To insure safe operation of an internal combustion engine, a control means, which detects the over-revolution of an engine and turns on an auxiliary device which prevents the over-revolution, has widely been used.
The control means is generally arranged so that when an engine over-revs, it delays or interrupts ignition and slows down the speed of revolution.
This control means is arranged such that it detects an output of an ignition coil of an ignition device or it detects an output of a signal coil for setting a ignition time, and when a speed of revolution of the internal combustion engine exceeds a set speed, it controls the primary current and delays or interrupts an ignition time and controls the speed.
The circuit of this control means includes a part of a circuit of an ignition device, so that an application of the control means to devices is not easy and is limited. Besides, when a speed of revolution is controlled by putting out the fire, it is difficult to smoothly light the fire again, so that a hysteresis of a speed control characteristic becomes higher resulting in irregularity of the revolution, and the speed of the revolution undesireably decreases, causing so called hunting.
To resolve these problems, new means have been introduced such as in Japanese Patent Provisional Publication No. 62-99675 and 63-9680.
Thses new means are arranged so that a speed control device is organized independently and is mounted to an ignition device, and the means detects the magnitude of the voltage induced in a primary winding of an ignition coil, which rises in proportion to the speed of revolution of an engine. When the voltage induced in the primary winding reaches a set voltage, the means detects that the speed of revolution has reached a set speed and delays ignition time of the ignition device to control the speed.
The speed control means mentioned above is organized totally separate from an ignition device, so that it can easily be mounted on the ignition device. Further, it is arranged to function only when the speed of revolutiuon of an engine is higher than a set speed, so that the engine revolves smoothly. However, the basic function of the speed control means is a so-called wave form detect charge-discharge voltage type analog control formula which is arranged such that a detected wave form is charged to a condenser and a discharge voltage is applied for its function, so that a controlled revolution becomes irregular because of a magnetic variation (wave form variation) and, thus theoretically, its function is unstabilized. Besides, a threshold point has to be set individually, depending an a dimension of an air gap between a flywheel and an ignition coil, so that installing the control means to an internal combustion engine requires a highly trained technician. Further, a lot of work such as setting the set point has to be done after installing the means to the engine, so that the installation procedure has become rather complicated work. Moreover, different types of control means are required, depending on a type and size of the engine, so that small numbers of many different types of control means have prepared for users, resulting in a higher price of the control means.
An object of the present invention therefore is to provide a method and a means, which stabilizes a control action by detecting the speed of revolution of an engine regardless of a magnitude of the voltage of a primary winding of an ignition device.
Another object of the present invention is to provide a method and a means which sends a drive signal which is previously set at a certain point regardless of the size of an air gap.
Another object of the present invention is to provide a means which can be mounted on any different type and size of an internal combustion engine.